U.S. Pat. No. 7,180,826, U.S. Patent Publication US2008/0179093-A1 and U.S. Patent Publication No. 2008/0271923-A1 are herein fully incorporated by reference as describing a flow throttling device (FTD) for use in signaling applications using pressure pulses in a constrained, moving fluid column. The FTD uses hydraulic power from the moving drilling fluid to actuate the FTD against the moving fluid column. A fraction of the drilling fluid is utilized in a pilot valve to control the FTD, resulting in greatly reduced energy required to operate the FTD.
In a typical borehole, a drilling fluid is pumped from the surface to the drill bit through a passage formed in the drillstring. The drilling fluid flows back to the surface within the annular space between the drillstring and the formation. Most drilling operations use “mud” as the drilling fluid, due to its relatively low cost and availability, readily controlled viscosity, and other desirable characteristics. The mud also lubricates the drillstring and drill bit and seals cracks and crevices in the surrounding formation by forming a mud cake. This “mud cake” also keeps the formation from caving in on the drill string.
In classical rotary drilling, fluid or drilling mud is pumped downward through a hollow drill string to the base of the hole where the drilling mud cleans the drill bit and removes or clears away the cuttings from the drill bit cutting surface. The cuttings are then lifted and carried upwardly along the well bore to the surface. Generally, the drill bit will contain jets which provide fluid flows near the bit and serve to increase the effectiveness of cuttings removal and thus enhance the rate of penetration (ROP) of drilling.
Several ROP enhancement patents describe the use of vibrating devices to cause the drill string to vibrate longitudinally and enhance ROP. Vibrations are transmitted through the drill bit to the rock face thus increasing the drilling rate somewhat. These devices were subject to a number of problems as noted in U.S. Pat. No. 4,819,745 to Bruno Walter.
More recently the drilling rate has been increased by periodically interrupting the fluid flow to produce pressure pulses in the fluid and in so doing, generating a water-hammer effect which acts on the drill string to increase the penetration rate of the bit. Axially movable valve members have provided a significant improvement over the known art that includes rotary valve arrangements which have been less prone to jamming and seizing as the result of foreign matter in the drilling fluid. There is, however, a requirement for higher pump operating pressures which have not been implemented on a majority of drilling rigs due to cost and other factors.
Another method relies on the interruption of the flow by a member operated by the reduction of the pressure due to the Bernoulli effect in the area under the movable member. A flow-pulsing apparatus described in U.S. Pat. No. 5,190,114 to Bruno Walter, relies on this Bernoulli effect. This design is sufficient when the drilling fluid is water. However at greater depths when the heavier drilling fluid is used, the restricting member stabilizes and the effectiveness of the system is reduced. This design uses smaller amplitude pulses at a higher frequency to reduce the solid to solid impact forces of prior art, but does not generate large enough amplitude forces to work in harder lithologies. Additionally, this design cannot work with higher bit weights above 20,000 pounds weight on bit (WOB). Mechanical design changes allow pulse frequency and amplitude to be adjusted.
Additionally, it has been demonstrated that significant increases in drilling rate can be achieved by maintaining a borehole pressure that is less than the formation pressure (in a technique referred to as “underbalanced drilling”). Underbalanced drilling is achieved by reducing the amount of weighting material added to the drilling mud or by using gas or foam for the drilling fluid. The problem with underbalanced drilling is that the entire open section of the hole is subject to low pressure, which reduces borehole stability and increases the risk of a “gas kick.” Gas kicks occur when the drill bit breaks into a region of higher gas pressure than the fluid column's mud weight pressure, causing gas bubbles to be entrained in the mud and rise toward the surface; the bubbles expand in volume as the pressure to which the bubbles are exposed drops when the bubbles rise in the borehole. If this gas kick goes unchecked through managed pressure drilling, a blowout may occur.
Another hydraulic system would provide a low-pressure region that is limited to the bottom of the borehole, with normal pressure controlling formation pressures higher up the hole. There have been attempts to achieve this condition using reverse flow bits; however, the bottom hole pressure reductions achieved with such bits have been relatively minor, i.e., less than 200 psi.
Another method of drilling uses interruption of the flow of the drilling fluid where the pressure of the drilling fluid forces the valve closed and/or opened. The pressures in the valve thus repetitively cycle it between an open and closed state. Drilling mud is fluid based and is thus substantially incompressible. Each time that the valve closes, the interruption of drilling fluid flow produces a “water hammer” pressure pulse upstream of the valve, due to the inertia of the flowing incompressible fluid against the closed valve. By continually cycling the valve between its open and closed positions, an axial force is applied to the drill bit by the repetitive water hammer pressure pulses. Since the frequency is relatively high (40 Hz or higher), the axial force is relatively small and it serves as more of an uncontrolled axial vibration on the bottom hole assembly (BRA) and does not substantially contribute to an improved drilling rate or efficiency.
It would be preferable to generate pulses in the drilling fluid having a pressure greater than 500 psi as a high amplitude, low frequency over the entire surface of the drill bits, since pressure pulses at these levels can generate forces that can fracture rock in the formation through which the drill bit is advancing and will greatly improve the efficiency of the drill bit by pushing the drill bit into the formation with substantially higher force than would be achieved using pump pressure and drill string weight alone. In addition, when the invention of the present disclosure creates a large amplitude, short duration pressure pulse by closing the pulsing fracturing device (PDD) in milliseconds, the application of the force at the bit is applied directly above the bit without the dampening effect of the drill string. Similarly, when the PDD opens, the stored fluid energy and pressure in the fluid column above the PDD is released in milliseconds, lifting the bit off the cutting face and generating a pressure shock wave through the jets clearing the cuttings away from the bit face, all of which, enhance the Rap. It is important to note that the quickness in which the PDD is closed and opened enhances the Rap since the axial forces are applied quickly. Additionally, Rap enhancement is optimized since the frequency and duration of the pulse is programmable on the surface. This allows the fluid column to reach a steady state flow pattern in between cycles.